1. Field of the Invention
The invention generally relates to the field of amplifiers, and more particularly to the common-mode feedback in differential or operational amplifiers.
2. Background Art
Operational amplifiers (op-amps) are circuits that produce a variety of different types of signals based on a set of inputs and the particular configuration in which they are used. The outputs of an op-amp sometimes are offset by a common-mode voltage. The common-mode voltage is usually a DC voltage that offsets the information-bearing AC signal. This common-mode voltage can be set by introducing a DC feedback path between the inputs and outputs of the op-amp. For example, a relatively simple op-amp can be introduced that compares the output common-mode voltage to a reference voltage, and based on that comparison sets a feedback signal that ultimately shifts the output common-mode voltage in a desired direction.
Including another feedback loop to set the output common-mode voltage of an op-amp, however, can have a number of disadvantages. For example, the loop used to set the differential-mode output voltage and the loop used to set the common-mode output voltage may have different stable ranges. Thus, conditions may exist under which the differential-mode output voltage is stable but the common-mode output voltage is not, and vice versa. Therefore additional compensation capacitors and resistors are required to make both loops stable. The additional circuitry used to create the common-mode feedback loop can consume additional power, take up additional space, and increase the response time to changes in the input common-mode voltage.
Thus, what is needed is an op-amp that allows the output common-mode voltage to be set without requiring additional circuitry that makes the op-amp conditionally stable and substantially contributes to the size, power consumption, and response time of the op-amp.